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, Thu, 23 May 2002 13:01:04
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As has already been stated, the Direct Ignition cartridge is just that: it drives the spark plugs directly, with no intervening coil, distributor or ignition leads.
Again, as mentioned, it is a highly stressed component, electrically-speaking, and the individual coils can eventually fail under heavy engine load, when the voltage required to create a spark is greatest.
However, that is not the whole story. The DI is also a sensing device, and uses the spark plugs to sense conditions inside the combustion chamber by measuring ionisation currents. These conditions include combustion chamber pressure and detonation. From these, the engine management system can take appropriate action, such as altering the ignition timing of an individual cylinder. In extreme cases, boost pressure (on turbocharged engines only) is adjusted too as a last resort (this is a function of the engine management system, not the DI, but it is triggered by the DI sensing adverse conditions). This system is designed to keep the engine running at its optimum for power and economy, as well as providing good starting at low temperatures and battery voltages, and with fouled plugs.
From the above, it will be apparent that the spark plug is a critical component in the functioning of the DI. Malfunctions can occur if the plugs are too old, the gap is too wide or the plugs are of the wrong type. The DI is *very* sensitive to the type of plug, and nothing but the correctly-specified plug, properly-gapped, will do (NGK BCPR7ES for turbos, BCPR6ES for non-turbos). People have reported here that, for example, the "equivalent" Bosch Platinum plugs have given good results, only to post again shortly afterwards that they're having problems.
Some people here advocate using dielectric grease on the plug boots to prolong life of the DI (reducing the possibility of arcing through a low-resistance path, perhaps?) and using smaller than the specified gap (I have found this to reduce low-speed performance). My own advice is to replace the plugs at every service (12,000 miles is specified for my UK-spec '96 Aero), even though Saab seems to say that plugs will last around 30,000 miles. A set for my car costs around £8 (US$12). I've done this as a matter of course on every car I've ever owned, even the very low-tech ones. If the correct plugs are hard to obtain in your locale, at least gap them often.
If you do insist on deviating from the correct plugs, remember that using non-resistive plugs (the "R" in "BCPR7ES" stands for "resistive" or "resistor") *will* cause to the DI to malfunction badly and damage it in no time.
However, if you remember all of the above and change the plugs with the correct type, at the correct intervals and keep them in good condition, you will be going a long way towards reducing your chances of being one of those who posts about a strange running problem, only to be told to "borrow a known good DI cassette" to see if that cures the problem, then shelling out upwards of £160 (US$235) for a new one.
posted by 80.2.252...
, Thu, 23 May 2002 13:01:04
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